Method of preparing encapsulated bichromal balls

ABSTRACT

Disclosed is a method of preparing encapsulated bichromal balls. The method involves forming capsules of electrophoretic particles encapsulated within a dielectric fluid such as a hydrocarbon solvent or siloxane oil that includes a small amount of a gelating agent. Bichromal balls are then formed by heating the capsules and applying an electric field. The capsules are then cooled while the electric field is maintained. Upon sufficient gelling, the electrophoretic particles are fixed within the gelled medium, i.e. the capsule. The bichromal gel produced detaches from the inside wall of the capsule, and thereby constitute the resulting encapsulated bichromal balls.

BACKGROUND

Illustrated herein are methods of preparing encapsulated bichromal ballsand the resulting balls produced thereby. The methods find particularapplication in conjunction with the use of such balls in displaydevices, such as photochromic displays, and substrates such as “electricpaper”, and will be described with particular reference thereto.However, it is to be appreciated that the embodiments are also amenableto other like applications.

Bichromal rotatable elements, such as bichromal balls, or beads assometimes referred to in the art, are tiny spheres, such as micron-sizedwax beads, which have an optical and an electrical anisotropy. Thesecharacteristics generally result from each hemisphere surface or sidehaving a different color, such as black on one side and white on theother, and electrical charge, i.e., positive or negative. Depending onthe electrical field produced, the orientation of these beads willchange, showing a different color (such as black or white) andcollectively create a visual image.

In this regard, the spherical particles are generally embedded in asolid substrate with a slight space between each ball and the substratebeing filled with a liquid so that the balls are free to rotate in achanging electrical field, but can not migrate from one location toanother. If one hemisphere is black and the other is white, each pixelcan be turned on and off by the electrical field applied to thatlocation. As a result, each pixel can be individually addressed, and afull page image can thus be generated.

For example, reusable signage or displays can be produced byincorporating the tiny bichromal beads in a substrate such as sandwichedbetween thin sheets of a flexible elastomer and suspended in anemulsion. The beads reside in their own cavities within the flexiblesheets of material. Under the influence of a voltage applied to thesurface, the beads will rotate to present one side or the other to theviewer to create an image. The image stays in place until a new voltagepattern is applied using software, which erases the previous image andgenerates a new one. This results in a reusable signage or display thatis electronically writable and erasable.

Conventional display devices, components for display devices, and themanufacture of such display devices and their components are describedgenerally in Sheridon, U.S. Pat. No. 5,604,027; Jacobson et al., U.S.Pat. No. 5,961,804; Jacobson et al., U.S. Pat. No. 5,930,026; Albert etal., U.S. Pat. No. 6,067,185; Crowley et al., U.S. Pat. No. 5,262,098;Sheridon, U.S. Pat. No. 5,344,594; and, Stefik, U.S. Pat. No. 5,723,204,the contents of which are incorporated entirely herein by reference.

Gyricon or bichromal balls or beads utilized in these displays aretypically produced by a spinning disc method and generally exhibit awide size distribution, i.e., from about 50 to about 200 microns. Theresolution of the Gyricon display is dependent on the bead size. TheGyricon beads used in current displays are from about 75 to about 110microns (μm). However, for many applications, it is necessary to usebichromal beads of much smaller size in order to achieve higherresolution as well as lower switching voltage. It has been foundextremely difficult to produce smaller beads while still maintainingacceptable characteristics such as bichromality, complementarity,sphericity, etc., and manufacturing yield.

Current Gyricon displays use a swollen elastomeric sheet in whichGyricon beads are dispersed. Recent investigation has shown that byencapsulating the Gyricon beads within an oil-filled capsule, the needto contain the swelling fluid as well as the need for the costlyelastomer can be eliminated. However, the encapsulating process alsoproduces some empty capsules, which must be completely removed in orderto prevent degradation of the optical contrast of the display. Thisfurther adds to the cost and complexity of manufacture. Accordingly,there is a need for an improved method for encapsulating bichromalballs, without the problems or degree of such problems, as currentlyknown.

The use of gelation or gelling techniques in the formation of bichromalballs is not generally known. Although the art refers to gel substratesfor retaining bichromal balls, i.e. U.S. Pat. Nos. 5,604,027; Re 37,085;this approach merely employs a gelled substrate which houses a bichromalball. That is, this description is not concerned with the actualformation of the bichromal ball itself. U.S. Pat. Nos. 6,488,870 and6,492,025 describe forming a shell about a bichromal ball, which incertain embodiments, may be formed by certain gelation techniques.However, these approaches are not relevant to a process of forming theactual bichromal ball or bead.

BRIEF DESCRIPTION

In accordance with one aspect of this disclosure, a method of formingencapsulated bichromal balls is provided. The method comprises providingelectrophoretic particles, and blending the particles with a dielectricfluid and an effective amount of a gelating agent. The method alsocomprises encapsulating the particles, dielectric fluid and gelatingagent within a capsule or shell structure. The method further comprisesheating the capsules to an elevated temperature and applying a field(i.e., electronic, magnetic, gravitational, etc.) to the capsules. Themethod also comprises cooling the capsules to thereby cause gellingwithin the capsule while the field is being maintained. Additionally,the method comprises removing the electric field and recovering theencapsulated bichromal balls.

As a result of this process, the bichromal gel produced in the capsulesdetaches from the inside capsule wall. The encapsulated bichromal ballsor beads formed by this process can rotate within the capsules uponapplication of an imaging electric field. The disclosure also includesthe encapsulated bichromal beads formed by this process and/or displaydevices utilizing the same.

These and other non-limiting aspects of the development are moreparticularly disclosed below.

DETAILED DESCRIPTION

The present disclosure relates to a method for producing encapsulatedbichromal Gyricon beads, or balls as sometimes referred to herein, bygelation of migrated colored electrophoretic (or magnetically polarizedparticles) inside a capsule or shell structure. The gelation of thebichromal balls inside the capsule or shell structure occurs utilizing aheating and cooling cycle while applying a field (i.e., electronic,magnetic, gravitational, etc.). The bichromal gel becomes detached fromthe inside capsule while cooling, thereby forming an encapsulatedbichromal ball.

The capsules prepared by the encapsulation process in accordance withthe present discovery include, in addition to one or more species ofcharged, colored electrophoretic particles, a hydrocarbon solvent orsiloxane oils with a small amount of gelating agent. By controlling theprocess parameters, the encapsulation process of the present discoveryis capable of producing small sized capsules (i.e., from about 2 toabout 750 microns, including from about 5 to about 200 microns,typically from about 10 to about 120 microns) with a narrow sizedistribution.

This process also produces encapsulated bichromal balls while minimizingthe number of empty capsules produced, thus enhancing the yield.

Also disclosed herein are particles that, used in electronic displaydevices, are encapsulated in a fluid such as a dielectric fluid or oil.The particles are hemispheric bichromal balls which have an optical andan electrical anisotropy due to each hemisphere surface having adifferent color (e.g., one hemisphere is white and the other hemisphereis black) and electrical charge. The bichromal balls are free to rotatewithin the capsules in response to an applied electrical field. Thebichromal balls are comprised, for example, of the followingillustrative materials: as the matrix, a polarizable material such as apolymer or a wax-like polyethylene was may be used; the white pigmentmay be titanium dioxide; and the black pigment may be magnetite (Fe₂O₃)or carbon black. Bichromal balls and their fabrication are described inU.S. Pat. Nos. 5,262,098; 5,344,594; and, 5,604,027, the disclosures ofwhich are totally incorporated herein by reference. In otherembodiments, the bichromal balls can be made with made with magneticanisotropy so that they are free to rotate within the microcapsules inresponse to an applied magnetic field.

Any fluid, or mixture of fluids, having dielectric properties may beused as the dielectric fluid to be encapsulated within the capsule shellalong with the electrophoretic particles. Examples of dielectric fluidsinclude partially fluorinated hydrocarbons, ISOPAR M or ISPOPAR L,polydimethyl siloxane oils, vegetable oils, etc., and combinationsthereof.

ISOPAR is the brand name for various grades of high-purity isoparaffinicsolvents with narrow boiling ranges, available from Exxon. Theexceptional purity of ISOPAR is the basis for such desirable propertiessuch as low odor, selective solvency, good oxidation stability, lowelectrical conductivity, and low skin irritation. The inherently lowsurface tension of ISOPAR also imparts superior spreadability toformulations utilizing ISOPAR. Other commercially available sources ofisoparaffinic solvents can be used such as Ashpar from Ashland Chemical,Inc., Columbus, Ohio. Furthermore, examples of suitable fluids includethose described in U.S. Pat. No. 6,067,185, the disclosure of which istotally incorporated herein by reference.

Some of the properties that the fluid should exhibit include chemicalcompatibility with the electrophoretic particles, capsule materials, aswell as the gelling agents used; low dielectric constant; high volumeresistivity; low viscosity; low toxicity; low water solubility; andhaving a similar density as well as refractive index to that of theelectrophoretic particles.

The electrophoretic particles may be composed of any suitable material,where the composition of the particles depends on their intended use.Examples of different types of electrophoretic particles suitable foruse herein are discussed in more detail below.

The amount and type of gelatin agent is also important. The gelatinagent must be of a type which is suitable in the dielectric fluid,particularly at encapsulation temperatures. Furthermore, the amount ofgelatin agent is important in that too much gelatin agent increases theviscosity to the extent that the whole mixture could gel. However, toolittle amount of gelatin agent fails to produce an effective product.Preferably, the ratio of gelatin agent to the dielectric fluids or oilis from about 0.1 weight percent to about 10 weight percent, includingfrom about 0.5 weight percent to about 5 weight percent, and from about1.0 weight percent to about 3.0 weight percent. This range willultimately depend upon the particular agent and system.

Numerous gelling agents may be used. The type of gelling agents that maybe used can be heat reversible gelling agents for hydrocarbon organicsolvents and hydrophobic organic or polymer liquids. The gelling agentsshould be able to dissolve in hydrophobic organic or polymer liquidswith warming and be able to form a nearly transparent gel uponsubsequent cooling. The gel should be stable in hydrophobic organic orpolymer liquids, but able to dissolve with a well-defined melting pointupon heating. The melting point of the gel depends on the polarity ofthe solvent and the concentration of the gelling agent. Preferably themelting point of the gelling agents is in a range of from about 45 toabout 70° C. and the gelling point should be in a range of from about 20to about 40° C.

An effective gelling or gelatin agent for use in this disclosureincludes trans-4-t-butyl-1-phenyl-cyclohexanol (i.e., compound 2),prepared as shown below.

Interestingly, only the diastereomer with an axial aryl group exhibitsany gellation ability. Compound 2 is a solid of low solubility innon-polar solvents. However, with warming the solid dissolves, and uponsubsequent cooling a nearly transparent gel results. The gel is stableto the presence of additional non-polar solvents, but will re-liquefywith a well-defined “melting point” upon heating. This melting pointdepends on the polarity of the solvent and upon the concentration of thegelling agent: less polar solvents are immobilized at lowerconcentrations, and higher concentrations of 2 always result in highermelting points.

A wide array of additional gelating agents may be used in conjunctionwith the present discovery. For example, other gelling agents withsimilar properties can be used, such as those described in, but notlimited to, EP 0207787; EP 0338738; U.S. Pat. Nos. 5,132,355; 3,960,514;5,417,287; 5,514,645; “Method of Gelling Hydrocarbons and FracturingSubterranean,” McCabe et al. Publications; “New Carbohydrate-BasedGelling Agents for Organic Solvents,” Tent et al. Cosmet. Toiletries(1977), 92(9), 39–40; and “Novel Family of Gelators of Organic Fluidsand the Structure of Their Gels,” Yih-chyuan Lin, Bechara Kachar, andRichard G. Weiss, J. Am. Chem. Soc. 1989, 111, 5542–5551; all of whichare hereby incorporated by reference. Non-limiting examples of othergelating agents includes 4-t-butyl-1-fluorinated arylcyclohexanolderivatives.

Several different types of encapsulation processes (i.e., complexcoacervation) can be utilized to produce capsules containing theelectrophoretic particles, the hydrocarbon solvent or siloxane oils andthe gelating agents. These are disclosed in more detail below.

Once encapsulated, formation of the bichromal balls is accomplished byheating the capsules to an elevated temperature, such as for example,from about 35° C. to about 100° C., including from about 35° C. to about100° C., and applying a field, preferably an electric field. This causesthe colored charged particles to migrate in an opposite direction insidethe capsules. Other fields, such as magnetic, gravitational, etc., canalso be applied. By cooling the capsules, such as to room temperaturewhile maintaining the field, the solvent phase gels, thereby yielding afixed bichromal bead. The field is then removed and the encapsulatedbichromal beads or balls are collected.

During this process, heating temperatures must be higher than thegelation temperature of the mixture of the dielectric fluids andgelation agent used. Additionally, the magnitude of the field should beat least sufficient to migrate the electrophoretic particles.

Upon completion of the process, surprisingly, the bichromal gel detachesfrom the capsule wall. Thus, the bichromal balls formed by this methodcan rotate in the capsules in response to an imaging electric field in afashion very similar to spinner-made Gyricon bichromal balls. Anadvantage of the rotation is robustness and longevity in comparison tocorresponding electrophoretic devices which require the continualmigration of many small particles. The encapsulated bichromal displaysproduced by the present disclosure exhibit much more enhanced imagestability in comparison to conventional electrophoretic displays.

It should be noted that the particle migration/gelation step can beaccomplished as part of the bichromal ball manufacturing process or aspart of a display fabrication process.

A wide array of electrophoretic particles can be used. The type ofparticles used will depend on the color of the display image that isrequired. The particles are submicron in size. The particles should becharged or capable of acquiring a charge, i.e. exhibit electrophoreticmobility. Particles that may be used include pigments such as, but notlimited to, titania, carbon black, etc., or dyed pigments, polymers orpigment/polymer composites.

Electrophoretic particles useful in the art of liquid toners orelectrophoretic displays may be used in the present discovery andcomprise composite particles of a pigment and a resin. Examples ofsuitable resins include polyethylene and polypropylene and theircopolymers, including ethylene-vinyl acetate copolymers and combinationsthereof. Examples of suitable pigments include rutile titania, anatasetitania, barium sulfate, zinc oxide, carbon black, Sudan blue, Hostapermpink, etc., and combinations thereof.

Additional examples of electrophoretic particles include, but are notlimited to, particles of a pigment and a resin. Examples of suitableresins include polyethylene and polypropylene and their copolymers,including ethylene-vinyl acetate copolymers such as the Elvax® I resinsavailable from E.I. DuPont Corporation, copolymers of ethylene and anα,β-ethylenically unsaturated acid selected from acrylic or methacrylicacid, where the acid moiety is present in an amount of from 0.1 to 20percent by weight, such as the Elvax® II resins available from E.I.DuPont Corporation, chlorinated olefins such as chlorinatedpolypropylene, including CP-343-1, available from Eastman Kodak Company,poly-α-olefins such as polyoctadecene and polyhexadecene, and the like.Within the particles, the resin is generally present in an amount offrom about 60 to about 95, and preferably from about 70 to about 90,percent by weight with respect to the pigment. Examples of suitablepigment materials include Raven® 5750 and Raven® 3500, available fromColumbian Chemicals Company, Mogul L, available from Cabot Corporation,Regal® 330 carbon black, available from Cabot Corporation, VulcanXC-72R, available from Cabot Corporation, Sudan Blue OS, available fromCiba-Geigy Inc., Hostaperm Pink E, available from American HoechstCorporation, Novaperm 3010, available from American Hoechst Corporation,Lithol Rubine DCC-2734, available from Dominion Color Company, Toner8200, available from Paul Uhlich & Company, Toner 8200, and the like.Generally, any pigment material is suitable provided that it consists ofsmall particles and that it combines effectively with the polymericresin material. Pigments, however, can affect the chargingcharacteristics of the particles, and a pigment of the desired colormust be chosen such that it imparts to the particles a charge of thedesired polarity and magnitude when mixed with a specific chargedirector. A specific pigment may result in a particle charging eitherpositively or negatively, depending upon the charge director used. Theparticles should have an average particle diameter of from about 0.1micron to about 10 microns, and preferably from about 0.5 to about 3microns, as determined by a Horiba CAPA-500 centrifugal particle sizeanalyzer, available from Horiba Instruments, Inc., Irvine, Calif., whichdetermines average volume particle diameter. The particles may bepresent in amounts of from about 0.5 to about 8, and preferably fromabout 2 to about 4, percent by weight of the composition.

In certain embodiments, the electrophoretic particles can be neatpigments, dyed (laked) pigments or pigment/polymer composites, or anyother component that is charged or capable of acquiring a charge.Typical considerations for the electrophoretic particle are its opticalproperties, electrical properties, and surface chemistry. The particlesmay be organic or inorganic compounds, and they may either absorb lightor scatter light. The particles may further include scattering pigments,absorbing pigments and luminescent particles. The particles may beretroreflective, such as corner cubes, or they may beelectroluminescent, such as zinc sulfide particles, which emit lightwhen excited by an AC field, or they may be photoluminescent. Finally,the particles may be surface treated so as to improve charging orinteraction with a charging agent, or to improve dispersibility.

One type of exemplary particle is titania. The titania particles may becoated with a metal oxide, such as aluminum oxide or silicon oxide, forexample. The titania particles may have one, two, or more layers ofmetal-oxide coating. For example, a titania particle may have a coatingof aluminum oxide and a coating of silicon oxide. The coatings may beadded to the particle in any order.

The electrophoretic particle is usually a pigment, a polymer, a lakedpigment, or some combination of the above. A neat pigment can be anypigment, and, usually for a light colored particle, pigments such as,for example, rutile (titania), anatase (titania), barium sulfate,kaolin, or zinc oxide are useful. Some typical particles have highrefractive indices, high scattering coefficients, and low absorptioncoefficients. Other particles are absorptive, such as carbon black orcolored pigments used in paints and inks. The pigment should also beinsoluble in the suspending fluid. Yellow pigments such as diarylideyellow, hansa yellow, and benzidin yellow have also found use in similardisplays. Any other reflective material can be employed for a lightcolored particle, including non-pigment materials, such as metallicparticles.

Useful neat pigments include, but are not limited to, PbCrO₄, Cyan blueGT 55-3295 (American Cyanamid Company, Wayne, N.J.), Cibacron Black BG(Ciba Company, Inc., Newport, Del.), Cibacron Turquoise Blue G (Ciba),Cibalon Black BGL (Ciba), Orasol Black (BRG (Ciba), Orasol Black RBL(Ciba), Acetamine Black, CBS (E.I. duPont de Nemours and Company,Wilmington, Del.), Crocein Scarlet N Ex (E.I. du Pont de Nemours andCompany) (27290), Fiber Black VF (duPont) (30235), Luxol Fast Black L(duPont) (Solv. Black 17), Nirosine Base No. 424 (E.I. du Pont deNemours) (50415B), Oil Black BG (E.I. du Pont de Nemours) (Solv. Black16), Rotalin Black RM (E.I. du Pont de Nemours), Sevron Brilliant Red 3B (E.I. du Pont de Nemours); Basic Black DSC (Dye Specialties, Inc.),Hectolene Black (Dye Specialties, Inc.), Azosol Brilliant Blue B (GAF,Dyestuff and Chemical Division, Wayne, N.J.) (Solv. Blue 9), AzosolBrilliant Green BA (GAF) (Solv. Green 2), Azosol Fast Brilliant Red B(GAF), Azosol Fast Orange RA Conc. (GAF) (Solv. Orange 20), Azosol FastYellow GRA Conc. (GAF) (13900 A), Basic Black KMPA (GAF), Benzofix BlackCW-CF (GAF) (35435), Cellitazol BNFV Ex Soluble CF (GAF) (Disp. Black9), Celliton Fast Blue AF Ex Conc (GAF) (Disp. Blue 9), Cyper Black IA(OAF) (Basic Blk. 3), Diamine Black CAP Ex Conc (GAF) (30235), DiamondBlack EAN Hi Con. CF (GAF) (15710), Diamond Black PBBA Ex (GAF) (16505);Direct Deep Black EA Ex CF (GAF) (30235), Hansa Yellow G (GAF) (11680);Indanthrene Black BBK Powd. (GAF) (59850), Indocarbon CLOS Conc. CF(GAF) (53295), Katigen Deep Black NND Hi Conc. CF (GAF) (15711),Rapidogen Black 3 G (OAF) (Azoic BIk. 4); Sulphone Cyanine Black BA-CF(GAF) (26370), Zamkbezi Black VD Ex Conc. (GAF) (30015); Rubanox RedCP-1495 (The Sherwin-Williams Company, Cleveland, Ohio) (15630); Raven11 (Columbian Carbon Company, Atlanta, Ga.), (carbon black aggregateswith a particle size of about 25 μm), Statex B-12 (Columbian Carbon Co.)(a furnace black of 33 μm average particle size), and chrome green.

Particles may also include laked, or dyed, pigments. Laked pigments areparticles that have a dye precipitated on them or which are stained.Lakes are metal salts or readily soluble anionic dyes. These are dyes ofazo, triphenylmethane or anthraquinone structure containing one or moresulphonic or carboxylic acid groupings. They are usually precipitated bya calcium, barium or aluminum salt onto a substrate. Typical examplesare peacock blue lake (CI Pigment Blue 24) and Persian orange (lake ofCI Acid Orange 7), Black M Toner (GAF) (a mixture of carbon black andblack dye precipitated on a lake).

A dark particle of the dyed type may be constructed from any lightabsorbing material, such as carbon black, or inorganic black materials.The dark material may also be selectively absorbing. For example, a darkgreen pigment may be used. Black particles may also be formed bystaining lattices with metal oxides, such latex copolymers consisting ofany of butadiene, styrene, isoprene, methacrylic acid, methylmethacrylate, acrylonitrile, vinyl chloride, acrylic acid, sodiumstyrene sulfonate, vinyl acetate, chlorostyrene,dimethylaminopropylmethacrylamide, isocyanoethyl methacrylate andN-(isobutoxymethacrylamide), and optionally including conjugated dienecompounds such as diacrylate, triacrylate, dimethylacrylate andtrimethacrylate. Black particles may also be formed by a dispersionpolymerization technique.

In the systems containing pigments and polymers, the pigments andpolymers may form multiple domains within the electrophoretic particle,or be aggregates of smaller pigment/polymer combined particles.Alternatively, a central pigment core may be surrounded by a polymershell. The pigment, polymer, or both can contain a dye. The opticalpurpose of the particle may be to scatter light, absorb light, or both.Useful sizes may range from 1 nm up to about 100 μm, as long as theparticles are smaller than the bounding capsule. The density of theelectrophoretic particle may be substantially matched to that of thesuspending (i.e., electrophoretic) fluid. As defined herein, asuspending fluid has a density that is “substantially matched” to thedensity of the particle if the difference in their respective densitiesis between about zero and about two g/ml. This difference is preferablybetween about zero and about 0.5 g/ml.

Useful polymers for the particles include, but are not limited to:polystyrene, polyethylene, polypropylene, phenolic resins, E.I. du Pontde Nemours Elvax resins (ethylenevinyl acetate copolymers), polyesters,polyacrylates, polymethacrylates, ethylene acrylic acid or methacrylicacid copolymers (Nucrel Resins—E.I. du Pont de Nemours, PrimacorResins—Dow Chemical), acrylic copolymers and terpolymers (ElvaciteResins, E.I. du Pont de Nemours) and PMMA. Useful materials forhomopolymer/pigment phase separation in high shear melt include, but arenot limited to, polyethylene, polypropylene, polymethylmethacrylate,polyisobutylmethacrylate, polystyrene, polybutadiene, polyisoprene,polyisobutylene, polylauryl methacrylate, polystearyl methacrylate,polyisobornyl methacrylate, poly-t-butyl methacrylate, polyethylmethacrylate, polymethyl acrylate, polyethyl acrylate,polyacrylonitrile, and copolymers of two or more of these materials.Some useful pigment/polymer complexes that are commercially availableinclude, but are not limited to, Process Magenta PM 1776 (Magruder ColorCompany, Inc., Elizabeth, N.J.), Methyl Violet PMA VM6223 (MagruderColor Company, Inc., Elizabeth, N.J.), and Naphthol FGR RF6257 MagruderColor Company, Inc., Elizabeth, N.J.).

The pigment-polymer composite may be formed by a physical process,(e.g., attrition or ball milling), a chemical process (e.g.,microencapsulation or dispersion polymerization), or any other processknown in the art of particle production. From the following non-limitingexamples, it may be seen that the processes and materials for both thefabrication of particles and the charging thereof are generally derivedfrom the art of liquid toner, or liquid immersion development. Thus anyof the known processes from liquid development are particularly, but notexclusively, relevant.

New and useful electrophoretic particles may still be discovered, but anumber of particles already known to those skilled in the art ofelectrophoretic displays and liquid toners can also prove useful. Ingeneral, the polymer requirements for liquid toners and encapsulatedelectrophoretic inks are similar, in that the pigment or dye must beeasily incorporated therein, either by a physical, chemical, orphysico-chemical process, may aid in the colloidal stability, and maycontain charging sites or may be able to incorporate materials whichcontain charging sites. One general requirement from the liquid tonerindustry that is not shared by encapsulated electrophoretic inks is thatthe toner must be capable of “fixing” the image, i.e., heat fusingtogether to create a uniform film after the deposition of the tonerparticles.

Further examples of suitable electrophoretic particles may be found inU.S. Pat. Nos. 4,880,720 and 6,249,271, col. 12, line 52 to col. 15,line 23; the disclosures of which are hereby incorporated in theirentirety.

The electrophoretic particles can include colloidal dispersions of finepowdered magnetic materials such as, but not limited to, ferrite,nickel, cobalt, iron, or various oxides thereof such as magnetite in asuitable polymeric binder. These particles will be able to migrate in afluid in response to the application of a magnetic field. For example,various powdered magnetic materials such as nickel, cobalt, iron orvarious iron oxides may be used; including the black oxide of iron,magnetite (Fe₃O₄). The particle size of the magnetic particlespreferably should be less than 0.25 micron in diameter to insure that atrue colloid is formed and, advantageously, may include particles ofless than 0.10 micron in diameter.

Although extremely fine particle sizes are obtainable by several knownprocesses (e.g.: vacuum deposition, condensation, and chemicalprecipitation or combination), grinding has been found to be a simpleand satisfactory method for obtaining a colloidal suspension of themagnetized iron particles. Dispersion is accomplished by grindingcommercially obtained powdered magnetite (a particle size ofapproximately 30 microns) in a ball mill in the presence of a grindingagent, which prevents agglomeration or welding of the minute particlesas grinding progresses. Generally, the grinding aid should comprisebetween 2 to 10 percent by weight of the metal particles and thegrinding process continues until the colloid solution is composed of 0.5to 10 percent by weight of suspended magnetic particles. Furtherreference can be found in U.S. Pat. No. 3,215,572, hereby incorporatedby reference.

The type of particles used will depend on the color of the desiredimage. These particles are generally submicron in size. They may behighly effective in scattering light (such as titania) while otherparticles may be highly absorptive (such as carbon black). The particlesshould be charged or capable of acquiring a charge of the desiredpolarity and magnitude when mixed with a specific charge controladditive. The pigments should effectively disperse the polymeric resinmaterial.

Charge control additives are optionally used to provide and/or enhancethe charge of electrophoretic particles to yield good electrophoreticmobility. Charge control additives suitable for the present discoveryinclude, but are not limited to, iron naphthenate and zirconium octoate,lecithin, barium petronate, and the like.

The encapsulation process is preferably by complex coacervation. Whencomplex coacervation is employed, cationic and anionic materials areused to form the capsule material. The cationic material and the anionicmaterial are oppositely charged polyelectrolytes which upon mixing willform a polyelectrolyte complex with low solubility in water, leading tocoacervation and formation of a protective microencapsulating shellaround each droplet. Suitable polyelectrolytes for the cationic materialand the anionic material include, for example, polyphosphates (e.g.,polyphosphorylated carbohydrates) and polycarboxylates (e.g.,polyacrylates and polymethacrylates), which may be combined withcationic polymers such as poly-N-ethyl-4-vinylpyridine orpoly-2,5-ionene bromide.

Other examples of anionic polymers are polysaccharides and theirderivatives such as acacia (gum arabic), carrageenan, agarose, alginicacid and salts thereof, heparin, hyaluronan, pectins and theirderivatives such as sodium amylosulphate. These may be combined withcationic materials such as chitosan or cationic cellulose derivatives,e.g., from hydroxyethylcellulose, such as Polymer JR (Union Carbide).Further examples of the anionic material are inorganic salts. Theinorganic salt may be, for instance, a polyphosphate. Inorganicpolyphosphate materials include, for example, alkali metal phosphates,phosphate glasses, alkali metal hexametapolyphosphates such as sodiumhexametaphosphates (trade name CALGON™). Other inorganic polyphosphatesinclude HYPHOS™ (Na₁₂P₁₀O₃₁, which contains 65 wt % P₂O₅), HEXATREN™ R,and HEXATREN™ N, including those disclosed in U.S. Pat. Nos. 6,488,870and 3,697,437, the disclosures of which are totally incorporated hereinby reference.

While complex coacervation is generally used to encapsulate theelectrophoretic particles as described herein, alternative encapsulationprocesses are also feasible. As the shell of the encapsulated Gyriconelements, a polymeric shell is typical. While any suitable polymermaterial may be used without limitation for the shell, the shell can bea polymer derived from two monomers that can be dissolved, respectively,in two mutually immiscible solvents (such as, for example, organicsolvents and water). This enables the polymer to be formed at theinterface of the two solvents via interfacial condensationpolymerization, as more fully explained below.

Shell polymers suitable for use with the embodiments described hereininclude those which may be formed in an interfacial condensationpolymerization process. Typical shell polymers include polyureas,polyurethanes, polyesters, thermotropic liquid crystalline polyesters,polycarbonates, polyamides, polysulfones, and the like, or mixtures ofthese polymers such as poly(urea-urethanes), poly(ester-amides), and thelike, which can be formed in a polycondensation reaction of suitablyterminated prepolymers or macromers with different condensationmonomers. For example, a preformed alcohol terminated urethaneprepolymer can be copolymerized with a diacyl halide to form apoly(ester-urethane) in an interfacial reaction, or an amine terminatedamide prepolymer can be copolymerized with a diisocyanate to produce apoly(urea-amide) copolymer. Epoxy monomers or oligomers such as Epikote819 can also be added in amounts of from about 0.01 percent to about 30percent to copolymerize into the shell as strengthening agents. Variouspolyfunctional shell monomers, such as triamines, triisocyanates, andtriols can be employed in small quantities of from about 0.01 percent toabout 30 percent as crosslinking agents to introduce rigidity andstrength into the shells. Shell polymers can also be formed by thereaction of aliphatic diisocyanates, such as meta-tetramethylenediisocyanate and a polyamine, reference for example the U.S. Pat. No.5,037,716, incorporated herein by reference in its entirety.

Particularly, the polymer shell material is comprised of a polyamide(from, e.g., diacid chloride and diamine monomers), a polyester (from,e.g., diacid chloride and diol monomers), a polyurea (from, e.g.,diisocyanate and diamine monomers), a polyurethane (from, e.g.,diisocyanate and diol monomers) or mixtures thereof. The diacid chloridemonomers and diisocyanate monomers may be dissolved in an organic phase,while the diamine and diol monomers may be dissolved in an aqueousphase.

Suitable shell monomers are usually selected from monomers where thenumber of chemical reacting groups per molecule is two or more. Thenumber of reacting groups per molecule is referred to as the chemicalfunctionality. An organic soluble shell monomer, which has afunctionality of 2 or more, reacts with an aqueous soluble shellmonomer, which has a functionality of 2 or more, via interfacialcondensation polymerization to generate the shell polymer in anembodiment of the present discovery.

The organic soluble shell monomer can include (1) diisocyanates such as,for example, toluene diisocyanate, nexamethylene diisocyanate,trans-1,4-cyclohexane diisocyanate, meta-tetramethylxylene diisocyanate(m-TMXDI), trimethylhexamethylene diisocyanate (TMDI), nexanediisocyanate (HDI), 4,4′-dicyclohexylmethane diisocyanate (Desmodur W),4,4′-methyldiphenyl diisocyanate and even diisocyanate prepolymers suchas polyether based liquid urethane prepolymer such as the Adipreneseries available from DuPont; XPS and XPH series which are toluenediisocyanate terminated polyethylene oxide prepolymers available fromAir Product, or (2) diacid chlorides (or, more generally, diacidhalides), such as, for example, sebacoyl chloride, terephthaloylchloride, phthaloyl chloride, isophthaloyl chloride, azeloyl chloride,glutaryl chloride and/or adipoly chloride. Examples of organic solubleshell monomers which have a functionality greater than 2 include1,3,5-benzenetricarboxylic acid chloride; Isonate 143L (liquid MDI basedon 4,4′-methyldiphenyl diisocyanate) purchased from The Upjohn Company;and tris(isocyanatophenyl) thiophosphate (Desmodur RF) purchased fromMobay Chemical Corporation.

Examples of monomers soluble in aqueous media and with a functionalityof two (2) include (1) diamines such as, for example, 1,6-hexanediamine,hexamethylenediamine, 1,4-bis(3-aminopropyl)piperazine,2-methylpipeazine, m-xylene-α,α′-diamine,3,3′-diamino-N-methyldipropylamine, 1,3-cyclohexanebis(methylamine),1,4-diaminocyclohexane, 2-methylpentamethylene diamine,2-methylpentanediamine (Dytek A) purchased from DuPont,1,2-diaminocyclohexane, 1,3-diaminopropane, 1,4-diaminobutane,2,5-dimethylpiperazine, piperazine, fluorine-containing1,2-diaminobenzenes purchased from PCR Incorporated, andN,N′-dimethylethylenediamine; (2) diols such as bisphenol A, otherbisphenols such as 4,4′-biphenol, 4,4-dihydroxydiphenyl ether, 3,3′- and4,4′-(ethylendioxy)diphenol, 3,3′- and 4,4′-(butylenedioxy) diphenol,4,4′-(hexafluoroisopropyldene)diphenol, 3,3′-and 4,4′-dihydroxydiphenylether, 3,3′- and 4,4′-biphenol, 4,4′thiobisphenols,4,4′-[1,3-phenylenebix(1-methylethylidene)]bisphenol,4,4′-bis(4-hydroxyphenyl)valeric acid and its alkylates,Iphenolphthalein and 3,3′- and 4,4′-methylenediphenols. Other diols whichmay be used include aliphatic diols such as: neopentyl glycol, ethyleneglycol, propylene glycol, butylenes glycol, diethylene glycol,dipropylene glycol, or mixtures thereof, or any other water solublecopolycondensation coreactant monomers/prepolymers. Other aqueoussoluble shell monomers having a functionality greater than 2 includediethylene triamine, bis(3-aminopropyl)amine, tris(2-aminoethyl)amine(TREN-HP) purchased from W.R. Grace Company, and the like.

More than one organic phase monomer can be used to react with more thanone aqueous phase monomer. Although formation of the shell entailsreaction in an embodiment between at least two shell monomers, onesoluble in organic phase and one soluble in aqueous phase, as many as 5or more monomers soluble in the organic phase and as many as 5 monomerssoluble in aqueous phase can be reacted to form the shell. In somepreferred instances, 2 monomers soluble in the organic phase and 2monomers soluble in aqueous phase can be reacted to form the shell.

Further, optional reaction aids such as catalysts or curing agents, maybe added to either of the solutions, if desired. For example, a shellcrosslinking agent such as Desmodur RF (Bayer) may be added to theorganic phase, if desired, in effective amounts of, for example, fromabout 0 to about 3 percent by weight of the monomers.

While several methods may be used to derive the encapsulating shellsurrounding the dielectric fluid and the Gyricon sphere, the process ofcomplex coacervation is typically used. Alternate processes, such asinterfacial condensation polymerization can also be used. As mentionedabove, interfacial condensation polymerization occurs at the interfacebetween two mutually immiscible solvents, usually an organic basedsolvent and an aqueous solvent (i.e., a water-based solution).

In one embodiment, the process involves forming the organicsolution/dispersion by dispersing the Gyricon spheres in a solutioncontaining an organic solvent, a monomer dissolved therein for example adiacid chloride or a diisocyanate monomer, and optionally alsocontaining a dielectric fluid. Preferably, the solvent is itself adielectric fluid such as an aliphatic hydrocarbon made by ISOPAR L orISOPAR M). However, the process can equally proceed using any organicsolvent whether dielectric or not, such as any hydrocarbon liquid, solong as if the solvent is not a dielectric fluid, a dielectric fluid isalso present in the solution.

The amount of the solvent compared to the amount of the Gyricon spheresin the organic phase dispersion should be such that the Gyricon spherescan each be coated with the dielectric fluid, e.g., by surface energyattraction of the fluid around the surface of the spheres. As anexample, from about 10 to about 95 percent by weight Gyricon spheres canbe added to the organic phase dispersion. The monomer level in theorganic phase may be from, for example, about 1 to about 100% (100%meaning neat monomer is the solvent).

The organic phase dispersion is next brought into contact with anaqueous phase solution containing a monomer that coreacts with themonomer dissolved in the organic phase, for example a diamine or diolmonomer. This solution is made by dissolving the monomer in water,preferably deionized water. The upper end of the monomer level in theaqueous phase is determined where the organic phase just barely becomesmiscible with the aqueous phase. The monomer level thus may be, forexample, from about 1 to about 50% monomer in aqueous solution.

Within the polymeric shell, the molar ratio of the organic solublemonomer to the aqueous soluble monomer is from about 1:1 to about 1:4,and preferably from about 1:1 to about 1:1.5.

In general, the interfacial condensation polymerization is conducted byfirst coating the Gyricon sphere with a first organic phase compositioncontaining a first monomer dissolved in the organic solvent, andoptionally a dielectric liquid, and subsequently exposing the coatedGyricon sphere to a second aqueous phase composition containing a secondmonomer dissolved in the aqueous solvent, whereby the first monomer andthe second monomer are made to react to form the encapsulating shell.

In one embodiment, this is accomplished by first mixing the Gyriconspheres into the organic phase composition, followed by exposing thecoated spheres to the aqueous phase composition. In this embodiment, theorganic phase is brought into contact with the aqueous phase, forexample by dropwise addition of the organic phase dispersion into thestirred aqueous phase solution. Upon contact, the monomers react (via acondensation reaction), forming a polymer skin around the droplets. As aresult, the polymer shell is formed around the core of Gyricon sphereand the dielectric fluid. The reaction generally occurs under agitation,for example stirring. The polymeric shell typically forms very quicklyupon contact of the two phases. However, the stirring and contact cancontinue for a period of, for example, about 1 minute to about 2 hoursor more, if desired.

In another embodiment, the coating with the organic phase and exposureto the aqueous phase compositions can be done through the use of anink-jet device, for example as detailed in U.S. application Ser. No.09/772,565, now U.S. Pat. No. 6,406,747, incorporated herein byreference in its entirety. Briefly, the method comprises first jetting aprecise amount of the organic phase composition onto the Gyricon sphere,which may be done by, for example, dropping the Gyricon sphere past theink jet nozzle. The coating will wet the entire surface of the Gyriconsphere due to surface energies. The Gyricon sphere coated with theorganic phase composition is then moved past an ink jet nozzle where itis jetted/sprayed with the aqueous phase composition, thereby causingreaction and the encapsulation.

In a still further embodiment, the coating with the organic phase andexposure to the aqueous phase compositions is conducted by dropping theGyricon sphere through a fog of the organic phase composition and thensubsequently dropping the coated Gyricon sphere through a fog of theaqueous phase composition. This method is also detailed in U.S.application Ser. No. 09/772,565, now U.S. Pat. No. 6,406,747,incorporated herein by reference in its entirety. Briefly in thisembodiment, separate fogs are created of both the organic phase andaqueous phase compositions, and the Gyricon sphere is made tosuccessively pass first through the fog of the organic phase and thenthrough the fog of the aqueous phase. The Gyricon sphere may be droppedthrough both fogs successively if the fogs are made to have the organicphase fog about the aqueous phase fog. Like the ink jet embodimentabove, the fog embodiment enables more precise control over the amountsof the compositions coated upon the sphere, thereby enabling moreprecisely sized encapsulated spheres to be derived and less waste ofmaterials.

The condensation reaction can be conducted at room temperature foreconomies. However, elevated temperatures may be used, if desired, toaid in the reaction. Following the encapsulation, the encapsulatedGyricon elements are collected by any suitable method known in the art.Following collection, the encapsulated Gyricon elements may be washed,if desired. Further details regarding materials and processes aredescribed in U.S. Pat. No. 6,445,490, herein incorporated by reference.

After the capsules have been formed, they are then heated while anelectric field is still applied. The field polarizes each particlewithin its respective capsule. The capsules are then cooled to causegelling to occur while the field is maintained. Upon sufficient gelling,the electrophoretic particles are then fixed within the gelled medium,i.e. the capsule, and the bichromal balls inside the capsule are therebyformed.

The present development provides a new method to produce encapsulatedbichromal balls by gelation of migrated colored electrophoretic ormagnetically polarized particles inside the capsules. The resultingbichromal balls are spherical or nearly spherical, separated from thecapsule wall, possess a dipole moment and are free to rotate in responseto the application of an electric field.

The size ranges for the capsules formed is from about 2 to about 350microns. The general size range is from about 5 to about 200 microns andthe typical size range is from about 10 to about 100 microns.

The present encapsulated bichromal balls may be dispersed into anysuitable medium which may be a liquid, a solid, or a gas, to form adisplay or a display surface. When these encapsulated bichromal ballsconstitute voltage sensitive members, the capsules may be dispersed inany medium across which an electrical field may be impressed. Mostcommonly, this medium will be a solid, with the particle or particlesdispersed in this solid while it is in a liquid phase. It may besubsequently hardened by chemical reaction, by cooling, or the like. Themedium may also be a liquid, or a slurry, consisting of a liquid andsolid particles, or solid particles whose purpose might be to immobilizethe capsules. Indeed, any medium might be used to contain the capsulesprovided that it does not damage the shell of the capsule or diffuseundesirable chemicals across the shell.

A series of trials were conducted to further investigate the presentdiscovery. A collection of microcapsules were made by the followingnon-limiting method:

To a 500 ml Morton reaction flask, 5 grams of gelatin (300 broom fromswine) and 110 ml of cold distilled water were added and the mixture wasstirred in a 60° C. water bath for about 0.5 hour. 10 grams of 5 wt %sodium polyphosphate was added and the pH value of the mixture wasadjusted to about 4.0 to 4.5 with acetic acid to induce formation of thecoacervate. After the coacervate was formed, 21.6 grams of a mixturecontaining small amount of dye (blue Nile) and white electrophoreticparticles of TiO₂ in ISOPAR M (about 5 wt %) with 0.216 grams of gellingagent trans-4-t-butyl-1-phenyl-cyclohexanol was then added. The mixturewas stirred at a temperature range from 60° to 30° C. for about 6 hoursand the capsules were formed. The capsule walls were crosslinked byreaction with glutaric dialdehyde and urea-formaldehyde. The capsuleswere washed with water, collected by filtration, and dried by a freezedrying process.

The release liner of an adhesive label (produced commercially by Xerox)was removed. The resulting sheet consists of an adhesive layer coated onwhite paper. Capsules prepared as described above were cascaded over theadhesive surface several times to yield a very uniform coating. Adisplay device was made by sandwiching the coated paper between twopieces of conducting glass having indium tin oxide conductive coating aselectrodes. Microscopic examination of the device showed that there wereno empty capsules.

The device was heated to a temperature of about 50° C. using a hotplate. Application of an electrical voltage about 300 to 500 volts ofone polarity caused the colored charged electrophoretic particles tomigrate in opposite direction within the capsules. The electric fieldwas removed after the device was cooled to room temperature to yield abichromal ball.

The operation of the display was demonstrated as follows. Application ofan electrical voltage (about 200 to 300 volts) of one polarity causedthe bichromal balls to orient preferentially in one direction (blue).Reversing the polarity of the electric field caused the bichromal ballsto orient in the opposite direction (white).

While particular embodiments have been described, alternatives,modifications, variations, improvements, and substantial equivalentsthat are or may be presently unforeseen may arise to applicants orothers skilled in the art. Accordingly, the appended claims as filed andas they may be amended are intended to embrace all such alternatives,modifications variations, improvements, and substantial equivalents.

1. A process of forming encapsulated bichromal balls, said processcomprising: providing electrophoretic particles; blending said particleswith (i) a dielectric fluid, and (ii) an effective amount of a gelatingagent; providing a shell material; forming capsules by encapsulatingsaid particles, dielectric fluid, and gelating agent within a shell ofsaid shell material; heating said capsules to an elevated temperature;applying a field to said capsules to separate the particles; coolingsaid capsules to thereby cause gelling within said capsule while saidfield is maintained; removing said field.
 2. The process of claim 1,wherein said dielectric fluid is selected from the group consisting ofpartially fluorinated hydrocarbons, isoparaffins, polydimethyl siloxaneoils, vegetable oils, and combinations thereof.
 3. The process of claim1, wherein said heating is performed to a temperature of from about 35°to about 100°.
 4. The process of claim 1, wherein said heating isperformed to a temperature of from about 35° to about 70°.
 5. Theprocess of claim 1, wherein said electrophoretic particles are pigmentsselected from the group consisting of titania, carbon black, andcombinations thereof.
 6. The process of claim 1, wherein saidelectrophoretic particles are composite particles of a pigment and aresin, wherein said pigment is selected from the group consisting ofrutile titania, anatase titania, barium sulfate, zinc oxide, carbonblack, Sudan blue, Hostaperm pink, and combinations thereof.
 7. Theprocess of claim 6, wherein said resin is selected from the groupconsisting of polyethylene, polypropylene, copolymers of polyethyleneand polypropylene, ethylene-vinyl acetate copolymers, and combinationsthereof.
 8. The process of claim 1, wherein said shell material isselected from the group consisting of polyphosphates, polycarboxylates,and combinations thereof.
 9. The process of claim 1, wherein saidparticles exhibit two colors.
 10. The process of claim 1, wherein saidelectrophoretic particles include fine powdered magnetic materialsselected from the group consisting of ferrite, nickel, cobalt, iron,oxides thereof, and combinations thereof.
 11. The process of claim 10,wherein said capsules have a size of from about 5 to about 200 microns.12. The process of claim 1, wherein said capsules have a size of fromabout 2 microns to about 750 microns.
 13. The process of claim 12,wherein said capsules have a size of from about 10 microns to about 120microns.
 14. The process of claim 1, wherein said field applied to thecapsules is a field selected from the group consisting of electronic,magnetic, and gravitational fields.
 15. A display device produced withthe encapsulated bichromal balls of claim
 14. 16. The encapsulatedbichromal balls produced by the process of claim
 1. 17. A process offorming encapsulated bichromal balls, said process comprising: providingelectrophoretic particles; blending said particles with (i) a dielectricfluid, and (ii) an effective amount of a gelating agent, wherein saidgelating agent is selected from the group consisting of materialscapable of gelling hydrophobic organic liquids; providing a shellmaterial; forming capsules by encapsulating said particles, dielectricfluid, and gelating agent within a shell of said shell material; heatingsaid capsules to an elevated temperature; applying a field to saidcapsules to separate the particles; cooling said capsules to therebycause gelling within said capsule while said field is maintained; andremoving said field.
 18. The process of claim 17, wherein said gelatingagent includes trans-4-t-butyl-1-phenyl-cyclohexanol and derivatives.19. A process of forming encapsulated bichromal balls, said processcomprising: providing electrophoretic particles; blending said particleswith (i) a dielectric fluid, and (ii) an effective amount of a gelatingagent; providing a shell material; forming capsules by encapsulatingsaid particles, dielectric fluid, and gelating agent within a shell ofsaid shell material; heating said capsules to an elevated temperature;applying a field to said capsules to separate the particles; coolingsaid capsules to thereby cause gelling within said capsule while saidfield is maintained; and removing said field; wherein said gelatingagent is present in a concentration of from about 0.1% to about 10% byweight of said dielectric fluid.
 20. The process of claim 19, whereinsaid gelating agent is present in a concentration of from about 0.5% toabout 5% by weight of said dielectric fluid.